Exemplary embodiments of the invention relate to a method for processing measured values from a nitrogen oxide sensor.
German patent document DE 10 2008 005 640 A1 discloses a method for determining a nitrogen dioxide concentration in an exhaust gas system of an internal combustion engine in which the output signals of two nitrogen oxide sensors are compared with one another. Here, the first nitrogen oxide sensor is arranged before and the second nitrogen oxide sensor after an exhaust gas treatment element with the capability of being able to convert nitrogen monoxide (NO) to nitrogen dioxide (NO2). This allows compensation for a different operating sensitivity with respect to NO and NO2, which is present with conventional nitrogen oxide sensors.
Exemplary embodiments of the invention are directed to a method that allows an accurate determination of a nitrogen oxide content in exhaust gas of an internal combustion engine.
With the method according to the invention, measured values from a first and a second nitrogen oxide sensor are processed. The first nitrogen oxide sensor is arranged upstream of a nitrogen oxide reduction catalytic converter in the exhaust gas system of a motor vehicle and the second nitrogen oxide sensor is arranged downstream of the nitrogen oxide reduction catalytic converter. With the method, measured values from the first and second nitrogen oxide sensors are recorded and compared with one another at least approximately at the same time, wherein the sensitivity of the first nitrogen oxide sensor and/or of the second nitrogen oxide sensor is changed depending on the result of the comparison. This allows compensation for undesirable sensitivity tolerances of the nitrogen oxide sensors, typically caused by production spread. The nitrogen oxide sensors are preferably such of the same design.
The sensitivity of the first and/or of the second nitrogen oxide sensor can, for example, be changed by changing the gain. Here, the change can occur directly at the output signal of the sensor; however, it can also occur as a result of signal correction at a control device to which the sensor or sensors are connected and which processes the signals of the sensor or sensors.
In an embodiment of the invention, the sensitivity of the first nitrogen oxide sensor and/or of the second nitrogen oxide sensor is changed in such a way that the first nitrogen oxide sensor and the second nitrogen oxide sensor have at least approximately the same sensitivity with respect to NO and/or NO2. Typically, the sensors provide an output signal that correlates with the sum of NO and NO2, i.e., with the concentration of nitrogen oxides (NOx), in the exhaust gas. The output signal is therefore an NOx concentration or content signal. The comparison of the measured values from the first and from the second nitrogen oxide sensor is preferably carried out under conditions with which the same concentrations or contents of NOx in the exhaust gas can be expected at the location of the sensors. When changing the sensitivity depending on the signal comparison, the signal from the sensor that indicates a lower NOx concentration, for example, can be increased to the signal of the sensor with the higher indication. Conversely, the signal from the NOx sensor with the higher measured value is reduced accordingly. However, the signals from the two sensors can adjusted to a mean value. In the cases mentioned, after carrying out the signal comparison and the sensitivity change, the nitrogen oxide sensors have at least approximately the same signals or the signals are corrected to an at least approximately equal value.
To avoid the influence of any cross-sensitivity of the nitrogen oxide sensors that may be present, in a further embodiment of the invention, the comparison is carried out at a time at which at least approximately equal concentrations of hydrocarbons (HC) and/or NOx are present in the exhaust gas at the point where the first nitrogen oxide sensor is installed and at the point where the second nitrogen oxide sensor is installed.
In a further advantageous embodiment of the invention, the comparison is carried out at a time at which the concentrations of hydrocarbons and/or ammonia (NH3) in the exhaust gas do not exceed specified limits at the point where the first nitrogen oxide sensor is installed and at the point where the second nitrogen oxide sensor is installed. This enables cross-sensitivity effects of the sensors with regard to HC and NH3 to be particularly effectively excluded. In particular, the limit values are set low. Preferably, the limit values lie below 20 ppm, particularly preferably below 10 ppm or even below 5 ppm.
In a further embodiment of the invention, the comparison is carried out at a time at which the exhaust gas temperature exceeds a specifiable temperature limit at the point where the first nitrogen oxide sensor is installed and at the point where the second nitrogen oxide sensor is installed. The temperature is preferably chosen such that the thermodynamic NO—NO2 balance lies predominantly on the side of NO. This is the case at temperatures above approximately 350° C. The comparison is therefore preferably carried out at a temperature limit of 400° C., in particular of 450° C. and particularly preferably at a temperature limit of 500° C. for the exhaust gas temperature. If a particulate filter is fitted in the exhaust gas system in question, then these conditions are usually fulfilled when a thermal regeneration of the particulate filter is carried out by burning off carbon black with oxygen. In a further advantageous embodiment of the invention, the comparison therefore takes place immediately following such a thermal particulate filter regeneration.
Advantageous embodiments of the invention are illustrated in the drawings and are described below. In doing so, the characteristics stated above and still to be described below can be used not only in the specified combination of characteristics in each case, but also in other combinations or in their own right without departing from the scope of the present invention.